Systems of this kind, designed to strike a target by means of a missile provided with a front antenna and a rear antenna, comprise an illuminator separated from the missile which simultaneously illuminates the target and the missile. The illuminator is associated with a tracking radar.
The illumination-wave signal reflected by the target is received by the front antenna of the missile and is detected coherently with the wave directly received by the rear antenna. The spectrum of the resulting signal contains the target echo at a frequency substantially proportional to its relative velocity. The Frequency-tracking means are then used to lock the receiver onto the target echo and to extract the data required for the automatic guidance of the missile.
Unfortunately, these systems are attended by considerable limitations which arise out of the need to use a rear channel on board the missile.
This is because the useful echo picked up by the receiver at the front of the missile may be masked by the illumination wave directly received by the side lobes of the front antenna. Although these two signals are not at the same frequency (the useful echo is affected by a frequency shift proportional to the velocity of the target relative to the missile), the parasitic echo received by the side lobes, affected by phase noise attributable to the illuminator and to the local oscillator of the receiver, may have a significant level at the Doppler frequency of the useful echo. In addition, this noise unnecessarily extends the spectrum received from ground echoes.
The conventional solution, in which the phase of the local oscillator of the receiver is controlled in dependence upon the illumination wave received by the rear antenna, makes it possible to obtain compression of the noise inherent in the local oscillator which, in addition, recopies the noise inherent in the illuminator.
In order to illuminate the rear channel in a sufficient volume of space, it is standard practice to feed a wide-beam ancillary antenna with a fraction of the power of the illuminator.
If the illuminator of the weapon system is independent of the radar, it is possible to couple the two transmitters with the same principal directional antenna while the illuminator alone feeds the ancillary antenna. Unfortunately, this solution is relatively complicated and inconvenient to use.
If it is the actual radar wave which simultaneously ensures illumination, the ancillary antenna is of necessity connected to the transmitter of the radar. It then impairs the performance of the principal antenna and, accordingly, the low-altitude performance of the radar by increasing the level of parasitic echoes attributable to the presence of the ground.
In a system where the tracking radar is a pulsed Doppler radar which is also used as illuminator for a pulsed Doppler autodirector, the problem is twofold:
it is no longer possible to decouple the ancillary antenna from the radar transmitter whose low-altitude performance is definitely impaired;
sampling of the signal received at the rear of the missile at the repetition frequency of the radar limits the bandwidth of the phase control of the local oscillator of the receiver and thus limits its effectiveness.